Here is another look at your question. We are getting new A321NEOs followed by XLRs with deliveries started in 23 and 24 respectively. Those will most likely take over the 757 flying and eventually grow it to include international and trans-cons and most likely some Hawaiian flying (my opine on the latter). We are getting new 737s at a prodigious rate, and they do some great flying, lots of variety as compared to what the Bus currently does. If you want seniority then go NB, with the 737 initially being better with the Bus fleet closing the gap in the near future. Also, remember, as we get the NEO / XLR expect the 757 fleet to start shrinking. The 767 fleet offers a lot as talked to above, just realize it will eventually start it's drawdown. Oh, there is the perpetual reserve of 777 / / 787 if one desires (consider moving to domicile for quality of life).

The above is just an opinion and is worth maybe 2 centavos.

Disclaimer, I'm a Bus guy.